rate problem

Water is withdrawn from a conical reservoir, 8 feet in diameter and 10 feet deep(vertex down) at the constant rate of 5 ft^3/min. How fast is the water level falling with the depth of the water in the reservoir is 5ft?

Ok so I think when you use the volume formula, you replace 5 for the final height and im not sure what else to do, or if that is even right. Please help!!